Many industries use precisely shaped parts. For example, conventional blades for use in gas turbine engines must be precisely shaped. Typically, precision investment casting or die-forging forms a metal starting blank into a blank having a blade shape. Generally, the blank includes a tip, a root region longitudinally spaced therefrom, and a midspan region extending between the tip and the root region. The tip may include an attached or integral shroud. A number of details must be machined into these portions of the blank. For example, a plurality of dove tails must be machined into the root region. These dove tails allow the blade to be secured to the rotor disk in the engine. In order to achieve acceptable engine performance, these dove tails must be formed within small tolerances. To accomplish this, there must be references from which to measure the configuration of the dove tails, and a fixture for holding the blank during machining.
Reference is made to U.S. Pat. No. 2,577,747 issued to Gibian and U.S. Pat. No. 3,818,646 issued to Peterson. However, the method taught in Gibian is not sufficiently accurate and the method taught in Peterson requires complex fixturing which is expensive to design, manufacture and maintain. Furthermore, a different fixture is needed for different sized and shaped blades.
Reference is also made to a method of providing references that entails encapsulating the blank in a block of material, such as a low melt alloy. The block of material is formed around the blank, so that the root region extends from the block. The sides of the block provide reference planes from which the configuration of the dove tails or other details can be determined with the necessary accuracy. However, encapsulating the blank requires a complex encapsulation tool. Furthermore, after machining the dove tails, the material must be melted off the blank and must be disposed of without causing environmental problems.
To overcome the problems above, it is known to use a blank having a sacrificial region and three locators. The sacrificial region extends longitudinally from the root region. The three locators are shaped and positioned to mate with corresponding features on a fixture. One of the locators is a conical shaped projection disposed at the tip. The other two locators are machined notches in the sacrificial region, one rectangular shaped notch and one wedge shaped notch. The conical shaped projection and the rectangular shaped notch both lie within a plane containing a stacking axis. The wedge shaped notch has a corner aligned with the root centerline, perpendicular to the stacking axis.
The fixture has a U-shaped base with spaced apart end walls to accommodate the blank. One of the end walls supports a clamping mechanism having a conical bore. The other end wall supports an abutment having rectangular and wedge shaped projections. Actuating the clamping mechanism moves the conical bore into contact with the conical projection on the blank. This forces the notched locators into contact with the corresponding projections on the abutment to thereby position the blank within the fixture. The surfaces of the wedge shaped projection make substantial contact with corresponding surfaces of the wedge shaped notch. Such contact helps to prevent the blank from rotating during machining and enables the blank to be secured within the fixture using only a minimal amount of clamping force in the longitudinal direction. Details, such as dove tails in the root region, can then be accurately machined into the blank to form a precisely shaped part.
The blank and fixture described above are disclosed in related copending application Ser. No. 08/641,251, "A Blank for Manufacturing Precisely Shaped Parts"; and Ser. No. 08/640,045, "A Fixture for Manufacturing Precisely Shaped Parts". This approach enables the part to be machined with improved accuracy. It also eliminates the need for fixtures with complex clamping mechanisms and does not require encapsulation with low melt alloy. The fixture is relatively simple to manufacture and can be easily modified to accommodate different size or shaped blades.
However, better fixtures are always being sought. For example, with the blank and fixture above, a significantly greater amount of longitudinal clamping force must sometimes be applied in order to prevent the blank from rotating during machining. However, greater clamping forces can distort the blank while it is secured within the fixture. This can result in misalignment of the subsequently machined features. Large clamping forces also present a risk of permanently distorting the blank. Thus, a fixture that does not require significantly greater than desired clamping forces in order to secure the blank is sought.